<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        /* 如果要所有属性都参与过渡，可以写all */
        /* transition:all 1s linear 0s; */
        /* all 不要滥用，影响效率 */
        div.d1 {
            width: 200px;
            height: 200px;
            border-radius: 0;
            background-color: red;
            /* 所有能过度的属性过渡 */
            transition: all 2s linear 0s;
        }

        div.d1:hover {
            width: 420px;
            height: 210px;
            border-radius: 30%;
            background-color: yellowgreen;
        }
    </style>
</head>

<body>
    <div class="d1"></div>
</body>

</html>